A plasma discharge, sometimes called a "high pressure plasma" is a gas which at high enough temperature (above 5000 K) becomes luminous and electrically conducting. These discharges are conventionally formed in electric arcs. Whereas the electrodes used to produce these plasma discharges are generally well cooled, they are nevertheless subject to chemical and thermal erosion from the reactive hot gases produced in an arc. This severely reduces electrode lifetime and arc stability.
In 1961, Reed, J. Appl. Physics 32, 821, first reported an atmospheric pressure electrical discharge produced without using electrodes by inductively heating a gas with high frequency electrical energy. This culminated a long history of experimentation with high frequency heating of low pressure gases. This discovery was later patented (U.S. Pat. No. 3,324,334). Shortly thereafter two more patents were granted (U.S. Pat. Nos. 3,467,471 and 3,521,959) for inventions first disclosing such a plasma discharge for use in optical emission spectroscopy for chemical analysis. Since 1964 the inductively coupled plasma (ICP) has been researched extensively as a source for optical emission spectroscopy in the laboratory and since 1975 has been accepted as a viable commercial tool for elemental analysis.
Owing to the ease of operation and desirable spectrochemical properties, ICP discharges for elemental analysis are now commonly operated at atmospheric pressure with argon, or a combination of argon and nitrogen, air, or oxygen. For these latter ICP discharges, argon supports the discharge and "coolant" nitrogen or other diatomic gas prevents the hot discharge from damaging the container walls.
In contrast to these diatomic gas-argon ICP discharges, no reports or patents are known to have previously appeared disclosing an inductively coupled non-argon plasma discharge (defined hereinafter) for spectrochemical analysis. Operation of a total air ICP, e.g., with a compressor used to supply the air results in an attractive reduction in operation costs for the gas supply and offers the convenience of never running out of tank gas. In terms of detection capabilities, the total air ICP has been found to be equal to or superior to the total argon ICP for the determination of calcium, for example. This indicates that for the analysis of calcium or other elements having similar ionization potentials, the air ICP can be used as a viable alternative to the argon ICP at a substantial reduction in operating costs.